1. Technical Field
The present invention relates to a lighting control circuit for vehicle lighting equipment and, more particularly, a lighting control circuit for vehicle lighting equipment constructed to control the lightening of a semiconductor light source that is formed of a semiconductor light emitting device.
2. Related Art
In the prior art, as the vehicle lighting equipment, the equipment using a semiconductor light emitting device such as LED (Light Emitting Diode), or the like as a light source is known. Also, the vehicle lighting equipment of this type is equipped with a lighting control circuit that controls the lighting of the LED.
As the lighting control circuit for the vehicle lighting equipment, for example, as shown in FIG.8, such a circuit is known that a converter circuit 1 is connected to a light emitting unit 3 consisting of light emitting diodes LEDs 11 to 1n, and a DC power is supplied from the converter circuit 1 to the light emitting unit 3, while controlling an output voltage of the converter circuit 1 such that a terminal voltage of a constant-current circuit 2 is maintained constant (see JP-A-2001-215913 (page 4 to page 5, FIG. 1)). According to this lighting control circuit for the vehicle lighting equipment, a ripple component can be removed from the output voltage of the converter circuit 1 by utilizing a current stabilizing action of the constant-current circuit 2.
In northern Europe, North America, and others, particularly in the district where an amount of solar radiation is small even in the daytime of winter, the driver is bound to turn on the headlamp of his or her car in the daytime, i.e., DRL (Daytime Running Light). Therefore, in the vehicle such as the car, or the like sold in these countries, the lamp control system that is used to turn on the headlamp in a dimmed lighting mode even in the daytime is employed (see JP-A-10-86746 (page 2 to page 5, FIG. 1)).
In turning on the vehicle headlamp as the DRL, the LEDs can be turned on in a dimmed lighting mode by supplying the power or the current, which is smaller than the power or the current in a full lighting mode, to the LEDs of the high-beam headlamp. For example, in the case where the constant-current circuit is utilized, if a current value is set to the current that is smaller than the current in a full lighting mode and then the set constant current is supplied to the LEDs, the LEDs can be turned on in a dimmed lighting mode.
Also, if a PWM (Pulse Width Modulation) signal, for example, is input directly to the LEDs and then supply/cutting-off of the power to the LEDs are repeated in accordance with the PWM signal, the LEDs can be turned on in a dimmed lighting mode. Also, if the PWM signal is input into a switching element of a switching regulator, which drives the LEDs by the constant current, and then the switching regulator is driven in accordance with the PWM signal, the LEDs can be turned on in a dimmed lighting mode. The PWM signal is defined as such a signal that has a frequency in a range of several hundreds Hz to several tens kHz, for example, and turns on/off the power (voltage/current) request at a particular duty ratio.
For example, in the case where the switching regulator is driven by the PWM signal that is input into the switching element and has a duty ratio of 50%, such switching regulator is caused to misunderstand that, when the duty ratio of the PWM signal is set to 50%, half of the particular constant current value that the switching regulator now controls has already been supplied. Then, the switching regulator supplies only half of the current required in a full lighting mode to the LEDs, and thus the brightness of the LEDs gives a quantity of light corresponding to such current. As the method of causing the switching regulator to misunderstand, the PWM signal is converted into a DC voltage and then this DC voltage value is reflected on feed-back control of the switching regulator. There are various methods such as the method of reflecting this DC voltage value on a current sensed value, for example. In this manner, the headlamp can be turned on in a dimmed lighting mode at any brightness by adjusting the duty ratio of the PWM signal to any value.